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We report the results on a new solder bumping technology for fine pitch organic substrates. To overcome the pitch limitation of the solder paste stencil printing method, pure molten solder is injected through a reusable film mask or directly injected without a mask on the pads of an organic substrate.
In the case of using a reusable film mask, targeted solder height over the solder resist (SR) is designed into the mask which has desirable thickness and holes size. In this study, three different solder bump heights such as 30, 50, 70 microns over SR were demonstrated for commercial organic substrates which have a pitch of 150 microns for 5,000 area array pads. To show the extendibility of the IMS bumping method to very fine pitch applications, 100 microns pitch bumping of 10,000 pads and 80 microns pitch bumping of 15,000 pads were demonstrated.
In mask-less IMS, the pure molten solder is directly filled into the opening volume of the SR; solidification of the solder under low oxygen leads to solder protrusions above the SR surface. For a 150 microns pitch commercial substrate, we demonstrated minimum height bumps of 15 microns over the SR. Since in this process there is no need to align a mask and a substrate, the mask-less IMS method lowers process cost and makes the process more reliable. By manipulating the opening in the solder resist, it is possible to enable variations in the height of the solder bumps.
Flux or formic acid is not needed during the solder injection of both described processes, but a low oxygen environment must be maintained.
In this paper, we will discuss laboratory scale processes and bump inspection data, along with a discussion of manufacturing strategies for IMS solder bumping technology for fine pitch organic substrates.